tyres: what's legal isn't always what's safe
The law states that you need to change your tyres when the tread depth reaches 1.6mm, but research has shown that safety becomes marginal long before this depth is reached. Andy Enright reports
Let's face it, for most of us it comes as a bit of a shock when our tyres have gone below the legal tread depth. The usual drill is to approach your car, spot that you're sporting a bit of a bald patch and then realise that the new laptop or weekend away that you'd promised yourself had gone down the pan, replaced by a visit to your local tyre depot. Joy.
The trouble is that by the time your tyres have reached the legal limit of 1.6mm tread depth across three-quarters of the contact patch and all around the circumference of the tyre shoulder, your performance and safety has already been seriously compromised. Independent research undertaken by MIRA Ltd (formerly the Motor Industry Research Association) reveals that braking performance deteriorates in wet conditions when tyre tread depth drops below 3mm.
Although the legal tread depth limit is 1.6mm, a threshold set by motoring experts some years ago, RoadSafe says there is now compelling evidence that motorists should check and change their tyres at 3mm. Road accidents where wet conditions are a contributory factor are on the increase, the road safety partnership says.
"A disproportionately high number of road accidents occur in the wet. Let's not forget last summer was one of the wettest on record and it's a well known fact that in wet conditions braking distances substantially increase. A third of all road accidents already occur during the summer months and with heavy downpours and flash floods becoming a permanent fixture of an English summer there is every possibility this figure could rise over successive years.
The cynic in me thinks that increasing the legal tread depth from 1.6mm to 3mm has to come off the back of pressure from tyre manufacturers who will suddenly increase sales as a result and a bit of background research into RoadSafe shows that 'vehicle component manufacturers' are part of its makeup. Rumbled, or so it would seem.
The truth is somewhere between the standpoints of penny-pinching motorists and profit-hungry tyre manufacturers and MIRA's testing procedures stand up to the closest scrutiny. Their research was carried out on four separate vehicles, a mid range family hatchback, an MPV, executive saloon and a high performance saloon car, measuring stopping distances at 50 mph in conditions that represented moderately heavy rain (0.5mm to 1.5mm water depth). The research discovered that tyres with a 3mm tread had a 25% better performance than those at 1.6mm. In terms of stopping distance this represents an extra 8 metres (25ft) in wet conditions. In one test, when a tyre with a tread depth of 8mm was compared to one with only 1.6mm, the stopping distance increased by 13 metres (42.25ft) - the equivalent to double the length of a typical living room. It doesn't take a genius to figure out that this could be the difference between a near miss and a crash with serious consequences. The recommended point for change is accepted Europe-wide as being 1.6mm but even ministerial cars in the UK have their tyres changed at 3mm.
Given that most tyres are supplied new with a 9mm tread depth, the calculation becomes clear. By replacing a tyre at 3mm instead of 1.6mm, your investment in rubber is netting you 6mm of tread wear rather than 7.4. By adding 12.3 per cent to your tyre bill, you're realising at worst a 25 per cent improvement in wet weather performance. Couched in those terms, running down to the legal minimum and beyond is the falsest of economies. When your family's safety is at stake, it pays to be a little more vigilant where tyres are concerned.